Can you spot the error, please? Why does the compiler think my function is undeclared? Thanks. -Rob
In .h file
-(int) getW:(int)xPosition;
In .m file
-(int) getW:(int)xPosition {
return (xPosition-58)/48;
}
in another procedure, the call to the function:
whichTile=[getW: xPosition] ; <----ERROR getW undeclared (first use in this function)
(xPosition and whichTile have been declared as integers, and in use earlier in the procedure). I tried it with (NSInteger), too (and a million other permutations!). Thanks for you help.-Rob
you have declared an instance method and called it without specifying the instance.
for example:
whichTile = [self getW:xPosition];
-or-
whichTile = [anObject getW:xPosition];
unlike other languages, self is not implicit when messaging.
Related
It seems that the default behavior in XCode is to silently allow redefinition of local variables if they are declared in a deeper scope, but throw an error or warning otherwise. For example, XCode produces an error for "Redefinition of 'var'" if it is redefined in the exact same scope:
- (void) doStuff
{
NSString *var = #"Hello World";
NSString *var = #"Goodbye"; // Error on this line
}
Similarly, if I have an ivar called 'var', and I try to re-declare 'var' in a local method, XCode will produce a warning for "Local declaration of 'var' hides instance variable" when I try to use it:
//MyClass.h
...
#interface MyClass : NSObject
{
NSString *var;
}
...
//MyClass.m
...
- (void) doStuff
{
NSString *var = #"Hello World";
NSLog(#"%#",var); // Warning thrown on this line
}
So far this is what I would expect. However, if var is redefined in a deeper scope, such as an if block or for loop, XCode allows it, and the outer declaration is silently ignored:
NSString *var = #"Hello World";
if (TRUE)
{
int var = 0;
NSLog(#"%d",var); //prints '0', No errors or warnings
}
NSLog(#"%#",var); //prints 'Hello World'
Why is the last example silently allowed, but the other two are caught? Is there some option or flag I can toggle in XCode so that an error or warning would also be created in the last example? If XCode won't catch it for me, is there some code I could write to make sure variables are never redefined? Or is it just my responsibility to make sure I'm not re-using my variable names?
In the build settings (Xcode 5 & 6, at least) you can set a warning for Hidden Local Variables to YES.
The last example is behavior that Objective-C inherits from standard C. A variable's scope is determined by the bracing level. It's been that way since the earliest days for C. It's called variable shadowing, and it's actually pretty useful in ensuring that code keeps working even in the face of API changes in system libraries.
As far as why it's allowed, but the earlier examples aren't, that's a consequence of how Objective-C implements instance variables. The instance variables are essentially treated as local variables of each of the class's methods. So when you declare a local variable in a function that shadows an instance variable, it gets flagged as an error. Basically the first and second cases are treated as equivalent.
To get a warning for these cases, set the LLVM warning option Hidden Local variables to Yes.
I'm trying to declare a function within another function. So here's part of my code:
ViewController.m
- (void)updatedisplay{
[_displayText setText:[NSString stringWithFormat:#"%d", counter]];
}
- (IBAction)minus1:(id)sender {
counter--;
updatedisplay();
}
ViewController.h
- (IBAction)minus1:(id)sender;
- (void)updatedisplay;
Which returned me the error of "Implicit declaration of function "..." is invalid in C99".
Result: http://i.imgur.com/rsIt6r2.png
I've found that people have encountered similar problem, but as a newbie I didn't really know what to do next. Thanks for your help! :)
Implicit declaration of function '...' is invalid on C99
You are not declaring a function; but a instance method, so to call it you must send it as a message to self;
[self updatedisplay];
EDIT
As #rmaddy pointed out (thanks for that) it is declared as instance method not class method. To make the things clear;
- (return_type)instance_method_name.... is called via 'self' or pointer to object instance.
+ (return_type)class_method_name.... is called directly on the class (static).
Problem
updatedisplay();
solution
[self updatedisplay];
cause
- (void)updatedisplay;
is a class method available for that class.So you have to call from the class to have the method available for you.
That is because you defined your function as a instance method, not a function.
So use it like
- (IBAction)minus1:(id)sender {
counter--;
[self updatedisplay]; // Change this line
}
write this way :
[self updatedisplay];
Do variables declared in methods get erased after the method call is done? Ie If i have the method "someMethod" and every time it is called i want to return a different NSString from a stack property will the method return the next object on the stack or will it keep returned the first index since x is erased at the end of the method call. I know if C that variables are erased after function calls, is it the same in objective c? Would using a seperate property for the variable x some this problem? Thanks
(Double) someMethod{
int x;
[self.stack objectAtIndex:x];
x++;
}
After reading the comments I tried creating a property to replace x and here is what I wrote but I get an error warning stating "local declaration of "_location" hides instance variable" What does this mean?
#property (nonatomic) int location;
#synthesize location=_location;
-(int) location{
if(!_location){
int _location = 0;
//warning is here
return _location;
}
_location++;
return _location;
}
(Double) someMethod{
int x;
[self.stack objectAtIndex:self.location];
x++;
}
Do variables declared in methods get erased after the method call is done?
Yes
Objective C methods are implemented "on top" of C functions, so the same rules apply. In particular, your code exhibits undefined behavior (reading of uninitialized variable).
To fix this issue, add an instance variable x in place of the automatic variable that your code snippet currently declares.
automatic is the "official" name of "stack" variables, i.e. variables that you declare inside your methods / functions.
Yes, the lifetime of local variables is limited to the time the enclosing function is executing (with exceptions for blocks, but you're not using those in this case).
Your counter x is probably best as an object property, if you want to maintain its value between calls to someMethod.
Answering to your question about the warning...
It is just saying that when you declare inside your if in method location()
int _location = 0;
this local variable has the same name as the property you created earlier
#syntenshize location = _location
Thus it gets confusing (for the programmer) to know which one he is using at the moment.
The compiler will understand that inside the if _location is an int ... and outside the if _location is your property.
So I am doing this to initialize my selector:
//In .h
SEL selectors[3];
//In .m
selectors[0] = #selector(rotate);
selectors[1] = #selector(discharge);
And here is the problem:
When I call this in my init method in Cocos2d like this:
[self performSelector:selectors[0]];
it works fine, but when I call this line of code in a method called moveThings which is invoked through the schedule ([self schedule:#selector(moveThings:)]) at the end of my init method in Cocos2d it gives EXC_BAD_ACCESS. What is the problem with scheduling things?
UPDATE:
I have found there is a problem with the rotate function (the function being stored in selector[0]). Here it is:
-(void)rotate:(ccTime)delta {
if (((CCSprite *)[creature objectAtIndex:0]).rotation < 360) {
((CCSprite *)[creature objectAtIndex:0]).rotation++;
}
else {
((CCSprite *)[creature objectAtIndex:0]).rotation++;
}
}
If I comment the contents of the method out it works fine when called through moveThings and init.
If I change the methods contents with:
((CCSprite *)[creature objectAtIndex:0]).rotation++;
It fails... But, again, I would like to state that all of these things do work if I call it in my init method, even call it twice in a row, but it will not work (except when I take out the contents of the rotate method) if I call it through the moveThings: method which is being invoke through the schedule method it fails.
Further update:
If I call:
((CCSprite *)[creature objectAtIndex:0]).rotation++;
In moveThings (which is being, as I've said before, invoked by the schedule:(SEL) method) it fails. Where as long as it is not invoked through a method that is the called by schedule it works.
The problem is that when you call performSelector there are only two options:
have your selector take no arguments and leave the ":" off the #selector(foo) definition.
have your selector take either one or two arguments which both must be an NSObject or subclass
it is the latter that is messing you up here I suspect.
Here are the three forms of performSelector:
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
You'll note that the withObject arguments are all of type id which is an Objective C object.
The selector you're trying to use takes a ccTime which is a float and not an Objective C object as it's param and that is why things are crashing:
-(void)rotate:(ccTime)delta;
One option is to make a wrapper method that takes a wrapped ccTime and unwraps it and calls the rotate:(ccTime) method:
- (void) wrappedRotate: (NSNumber*) inDelta
{
[self rotate: [inDelta floatValue]];
}
then use
selectors[0] = #selector(wrappedRotate:);
and then call via:
[self schedule: #selector(moveThings:)]); // cocos2d schedule
...
- (void) moveThings: (ccTime) dt
{
[self performSelector: selectors[0] withObject: [NSNumber numberWithFloat: dt]];
...
}
One reason you are getting confused is because Cocos2d is using #selector in somewhat more complicated ways (see CCScheduler.m in the CCTimer::initWithTarget:selector:interval: and CCTimer::update: method in particular).
disclaimer: code typed into SO so not checked with a compiler, but the essence of what you need should be here.
One problem for sure is that you are using a variable declared inside a .h while initializing it inside the relative .m. According to the linking I'm not sure that just one variable selectors will exist (so that different files that include .h will have different versions).
First of all I suggest you to try adding the keyword extern to have
extern SEL selectors[3];
to tell your linker that it is initialized inside the relative .m and to use just that one.
I think your problem stems from your method definition which is - (void)rotate; and not - (void)rotate:(ccTime)dt;
You should adjust your selectors likewise.
If your method does not have any arguments then do not use a colon in your selector call.
// Requires #selector(foo:)
- (void) foo:(id)sender;
// Requires #selector(foo)
- (void) foo;
i've got this method:
-(void)reportAchievementIdentifier: (NSString*) identifier percentComplete: (float) percent
now I want to call this method in another method like:
[self thisMethod];
But how can I do this with a method that has local declarations in it?
thank you
It doesn't matter if a method has local declarations in it, that's completely normal. Have you tried calling [self thisMethod];? Does it crash?